10 November 2004

Promiscuous Females Make For Competitive Sperm

by Kate Melville

Howard Hughes Medical Institute researchers have shown that when females are more promiscuous, male genes have to work harder. They determined that a protein controlling semen viscosity evolves more rapidly in primate species with promiscuous females than in monogamous species. The research - published in Nature Genetics - demonstrates that sexual competition among males is evident at the molecular level, as well as at behavioral and physiological levels.

Lead researcher Bruce Lahn and his team studied semenogelin, a major protein in the seminal fluid that controls the viscosity of semen immediately following ejaculation. In some species of primates, it allows semen to remain quite liquid after ejaculation, but in others, semenogelin molecules chemically crosslink with one another, increasing the viscosity of semen. In some extreme cases, semenogelin's effects on viscosity are so strong that the semen becomes a solid plug in the vagina.

Lahn believes that such plugs might serve as a kind of molecular "chastity belt" to prevent fertilization by the sperm of subsequent suitors. Lahn and his colleagues began by sequencing the SEMG2 gene in humans, chimpanzees, pygmy chimpanzees, gorillas, orangutans, gibbons, macaques, colobus monkeys, and spider monkeys. These species were chosen because they represent all the major mating systems, including those in which one female copulates with one male in a fertile period; those in which females copulate highly promiscuously; and those in which mating practices fall somewhere in between. "When we plotted data on the evolution rate of the semenogelin protein against the level of female promiscuity, we saw a clear correlation whereby species with more promiscuous females showed much higher rates of protein evolution than species with more monogamous females," said Lahn. The researchers measured protein evolution rates by counting the number of amino acid changes in the protein, then scaling it to the amount of evolutionary time taken to make those changes.

"The idea is that in species with promiscuous females, there's more selective pressure for the male to make his semen more competitive. It's similar to the pressures of a competitive marketplace. In such a marketplace, competitors have to constantly change their products to make them better, to give them an edge over their rivals. The finding constitutes the first specific evidence that different levels of sexual competition produce different genetic effects. The genes have to adapt faster for any given male to gain an edge over his competitors," said Lahn.